The chain also redesigned the menu and explained that money from the levy would go to charity.

Experts said more research was needed to pin down what measures worked.

Consuming too many sugary soft drinks has been linked to a higher risk of serious health problems such as obesity, type-2 diabetes, heart disease, stroke and tooth decay.

Sugar tax

To help tackle obesity, the UK government is introducing a tax on high-sugar soft drinks such as Coca-Cola, Pepsi and Irn-Bru in April 2018 – and Jamie Oliver had been vocal in his support of the plan.

This study, in the Journal of Epidemiology and Community Health, analysed sales of sugary non-alcoholic beverages at 37 of Jamie Oliver’s national chain of restaurants after a 10p levy was introduced in September 2015.

Low-sugar fruit spritzers (fruit juice mixed with water) were also added to the menu, which clearly explained why the levy was being introduced.

Image captionJamie Oliver urged the government to be bold on a sugar tax

After 12 weeks, sales of sugary drinks per customer had declined by 11%, and after six months they had gone down by 9.3%.

But the study did not look at any other restaurant chains to compare sales figures.

The study also showed there was a general decrease in the number of soft drinks sold per customer, including diet drinks and bottled waters.

The researchers, from the London School of Hygiene and Tropical Medicine (LSHTM) and the University of Cambridge, said more people could have chosen tap water, but these figures had not been recorded.

Sales of fruit juices had increased by 22% six months after the changes were introduced.

Changing behaviour

Prof Steven Cummins, lead study author and professor of population health at LSHTM, said: “A small levy on sugar-sweetened drinks sold in restaurant, coupled with complementary activities [such as redesigning the menu], may have the potential to change consumer behaviour.”

But he said it was not possible to say that the price increase alone had caused the decline in sales of sugar-sweetened drinks.

There was also no separate data on what adults and children ordered.

Image captionCustomers may have ordered tap water instead of sugary drinks

Prof Kevin McConway, emeritus professor of applied statistics at the Open University, said it was plausible that the levy “played an important role” but he also called for “more investigation, in other restaurants, and with a longer follow-up period, to try to pin down more clearly what really works”.

Prof Susan Jebb, professor of diet and population health at the University of Oxford, said the findings were “encouraging news for public health”.

But she said there was a disappointing lack of data on alcohol sales, which could have increased over the same period.